The invention relates to a method for navigating a vehicle, in which sensors are used to ascertain a vector for a distance covered from direction and length, a plurality of vectors are used to determine a position of the vehicle, and the position is depicted on a map position, where, under prescribed conditions, the direction ascertained by the sensor is corrected using a direction ascertained on the basis of the map.
Navigation in which the current position of the vehicle is determined by continuously ascertaining the direction and magnitude of a movement starting from a starting point is also called xe2x80x9ccompound navigationxe2x80x9d or dead reckoning. Provided that the direction and the distance covered in this direction can be determined relatively accurately, relatively accurate position determination is also obtained. If such position determination is carried out at a frequency of 5 Hz, then the individual vectors, i.e. the sections of road whose direction and length are being determined, with an order of magnitude in the meter range are small enough to enable virtually continuous position determination to be performed.
However, the sensors required to determine the direction and distance predominantly operate with a certain degree of error. This is particularly valid for the direction sensors, which are predominantly in the form of gyroscopes today. A gyroscope outputs an output signal, called drift, with a DC voltage at a predetermined level so long as the vehicle is moving in a straight line. When the direction of the vehicle changes to the left or to the right, the voltage rises or falls on the basis of the angular speed during the change. By integrating this signal over time, it is possible to ascertain the angle of the direction change.
However, since gyroscopes normally have a drift error, it is entirely possible that the navigation system will assume a direction change even though no direction change has actually taken place. In the same way, an incorrect angle can be ascertained for a direction change.
To remedy this problem, a control option is used. This involves tracing the path of a vehicle on a map. In this case, the map is stored in a database or in another suitable data format. In this context, it is assumed that the vehicle is situated on a road if such a road is situated close to the position determined by compound navigation. Such a procedure is also called map matching, and the part of the navigation system which carries out this depiction is called the map matcher.
By combining compound navigation with the control using the map positions, relatively high degrees of accuracy are achieved when determining the position of the vehicle. Added to this is the fact that the series of positions determined on the map can be used to draw conclusions as to whether the direction sensor is operating with the required degree of reliability, for example. Furthermore, when the direction of the vehicle on the map has been determined, this direction can be used to determine the next vector. Such resetting of the vehicle direction to the value obtained by map positioning is carried out, for example, whenever at least 3 sec. have elapsed since the last direction change.
However, this procedure has the disadvantage that resetting to the direction ascertained using the map can create a new error which, under some circumstances, significantly impairs the positioning result.
The invention is based on the object of improving continuous position determination.
This object is achieved for a method of the type mentioned in the Background of the Invention section above by virtue of the feature that, if a direction difference arises between the sensor and the map, the direction difference or the variables on which it is based are subjected to a plausibility check.
In this case, it is first assumed that there is no need for correction if there is a match between the direction determined by the sensor and the direction determined using the map. If such a direction difference has arisen, then there appears to be a need for correction. In this case, however, the new direction used for compound navigation is not merely set equal to the direction which has been ascertained using the map, but rather a check is first carried out to determine whether it is at all possible that the vehicle has taken this direction. If appropriate, a check is carried out to determine whether there is a particular degree of probability of this. Only if this condition is satisfied is the correction made.
One preferred procedure for the plausibility check is that a possible maximum error is estimated and a correction is then not made if the estimated maximum error is smaller than the direction difference. In this context, it is first assumed that the error can primarily be attributed to the error of the direction sensor. In this case, it is then possible to estimate the maximum possible level of this error. If the direction difference is still greater than this maximum error, then there is a high degree of probability that the error has not or has not only been caused by the direction sensor, but rather the depiction on the map is possibly erroneous.
The maximum error is preferably formed by a product comprising an error value and the time since the last correction, possibly plus an estimated error value comprising the last direction correction. As mentioned above, the direction sensor outputs a signal which deviates from a prescribed mean value if and so long as the vehicle is executing a direction change. In the event of an error, the signal deviates from the mean value by the error value even when the vehicle is not executing a direction change, but rather is traveling in a straight line. The maximum error can then be no larger than the integral of this error value over time, and possibly of an initial error which may have arisen during the last correction and can be prescribed as an estimate value, for example. If the direction difference is greater, then the error does not or does not just lie with the direction sensor. In this case, a correction must not be made.
The error value is preferably ascertained from previous corrections. The result of this is dynamic matching of the error value. In this context, allowances can be made, for example, for the fact that the error value changes over time, for example on account of changing operating conditions. Many direction sensors have a certain degree of temperature dependency.
Preferably, the error value is filtered. This allows the error value to be ascertained even more effectively. Since, under some circumstances, direction determination using the map may likewise have errors from time to time, such xe2x80x9coutliersxe2x80x9d have only a reduced influence on the error value actually used later.
It is also advantageous if the map is searched for alternatives for which the direction difference is smaller. In many cases, the error arises simply as a result of the map matcher accessing an incorrect database sector. When xe2x80x9cmapxe2x80x9d is mentioned in this context, this always refers to the form stored in the form of data records or database sectors. When an incorrect database sector is accessed and an error is produced as a result, it is possible to look in adjacent database sectors to determine whether a better match can possibly be found there. Adjacent database sectors are those containing coordinate ranges which are adjacent to one another in reality. The sectors themselves need not be adjacent in physical or address terms.